Material handling vehicles, and particularly those known as reach trucks, have a lift mast mounted thereon which is longitudinally movable on the vehicle between spaced apart locations. Typically, the lift mast assembly is guided for longitudinal movement by chains, rollers, and the like engaged with spaced apart frame side members which extend in a longitudinal direction relative to the longitudinal axis of the vehicle. The guide rollers and chains engage the spaced apart frame side members in a manner so that the mast is movable along the vehicle axis and to some degree in directions transverse the vehicle axis to accommodate manufacturing tolerance build up. Often excessive play in directions transverse the movement of the lift mast assembly along the frame side members will permit cocking, skewing and other movements of the lift mast which are detrimental to the performance and life of the chains, rollers, and associated componentry. Representative examples of different ways of connecting the lift mast assembly to the vehicle for longitudinal movement along spaced apart frame members as discussed above are shown in U.S. Pat. No. 2,320,601 to R. C. Howell dated June 1, 1943, U.S. Pat. No. 2,667,985 to H. D. Woughter dated Feb. 2, 1954, U.S. Pat. No. 2,997,194 to A. E. R. Arnot dated Aug. 22, 1961, and U.S. Pat. No. 3,240,372 to J. E. Joyce et al. dated Mar. 15, 1966.
In order to move the lift mast assembly along the spaced apart side members of the vehicle frame, a suitable drive mechanism must be provided. Heretofore mentioned U.S. Pat. No. 2,667,985 to H. D. Woughter dated Feb. 2, 1954 utilizes drive chains, sprockets, and an electric motor to move the lift mast longitudinally relative to the vehicle, and aforementioned U.S. Pat. No. 2,320,601 to R. C. Howell dated June 1, 1943 utilizes a hydraulic jack connected between the vehicle frame and the lift mast assembly for propelling the lift mast along the spaced apart side members and longitudinally relative to the vehicle. Each of these drive arrangements are complicated, expensive, noisy, require a substantial amount of space on the vehicle, and do not precisely and smoothly position the lift mast along the side frame members.
Screw drives have been utilized to elevationally move lifting devices. Typically, screw drives are connected between a fixed member and a movable member and elevate the movable member in response to rotation of the screw. Examples of conventional lifting apparatuses having screw drives are shown in U.S. Pat. No. 2,663,929 to L. M. Carpenter dated Dec. 29, 1953, and U.S. Pat. No. 3,309,060 to J. Villars dated Mar. 14, 1967. In each of these patents, the screw drives are subjected to side loading forces which are transferred from the load being lifted to the lifting device and to the screw shaft. As a result, these side loading forces cause premature wear and ultimately failure of the screw and associated screw drive componentry. As a result, screw drive arrangements for lifting loads have seen only limited use and limited success.
Because a screw drive is capable of accurately and precisely positioning the driven element along its length, it is a desirable way of moving a lift mast assembly along the longitudinal vehicle axis. However, in order to be able to successfully utilize a screw drive, the problems associated with side loading the screw drive must be reduced to an acceptable level. The solution to this problem is complicated even more due to the fact that a controlled amount of movement of the lift mast assembly transverse the longitudinal axis of the vehicle must be provided to accommodate manufacturing and assembly tolerance stack up, and the like.